So a app. wouldn't cause a system wide freeze, didn't think so, just asked to make sure.Have you tried running firefox in a terminal and see if it spits any output if it's just the app that freezes?
An app could possibly cause the whole system to come down but it would be very unlikely. I would say that most of the time if the whole system crashes then there is a problem with the kernel or one of its modules that your hardware has loaded.
Could also be bad RAM, might be worth running memtest. But yer it's most likely kernel issues as above.